As is generally known, polytetrafluoroethylene has a melting viscosity of 10.sup.11 poises or thereabouts even at a temperature of which is higher than its melting point (327.degree. C.), and this melting viscosity is far higher than a viscosity of 10.sup.3 -10.sup.4 poises at the molding temperature of common plastics.
On that account, it is not possible to extrude or roll polytetrafluoroethylene resin in the same manner as in the case of common plastics which can be extruded or rolled in a soft or fluid state by heating. Therefore, molded articles of polytetrafluoroethylene resin (hereinafter called PTFE resin) such as tubes and rods are produced by a paste extrusion molding technique.
In this paste extrusion technique, unsintered PTFE resin powder is usually incorporated with an extrusion aid such as solvent naphtha, illuminating kerosine or toluol in order to impart fluidity to said unsintered powder, the resulting mixture is preformed to obtain a preform of a cylindrical shape, the preform (billet) thus obtained is fed into a cylinder of an extrusion machine and extruded therefrom by application of ram pressure. According to the paste extrusion technique, unsintered particles of PTFE resin undergo plastic deformation with the assistance of the extrusion aid and are extruded through a die fitted to the cylinder head, whereby an unsintered PTFE resin molded article formed to shape in continuous form is obtained. The unsintered PTFE resin molded article is then inserted into a long cylindrical oven, wherein said molded article is dried in the drying zone kept at about 100.degree.-250.degree. C. to remove the extrusion aid therefrom, and then sintered in the sintering zone at a temperature above the melting point of PTFE resin, for example, 360.degree.-380.degree. C., followed by cooling, whereby a compact sintered PTFE resin molded article having a sufficient mechanical strength is obtained.
However, when tubes or rods of continuous length were extrusion molded in the manner as mentioned above, in the sintering step subsequent to the drying step there were observed such drawbacks as an outside dimension of the resulting molded article became inhomogeneous, the section of tube or rod was out-of-round, or the above-mentioned tubes or rods of continuous length were not sintered linearly and warped in the lengthwise direction. A further drawback observed was that the molded articles are not obtained with the desired dimensional accuracy of mm unit. This tendency was pronounced particularly in cases where the molded articles were large in outside diameter.
The products produced in this manner, such as tubes or rods, not only depreciated their commercial value but also caused such inconvenience as will be mentioned below when a steel pipe is lined inside with said tube.
As an example of the process for producing a steel pipe lined inside with PTFE, there is a process wherein the steel pipe is lined inside with an extrusion molded PTFE tube by drawing said tube into said pipe. In this case, there must be a clearance between an outer periphery of the lining tube and an inner periphery of the steel pipe to be lined therewith, and usually this clearance is appropriately about 2% of an inside diameter of the steel pipe. Because every section of PTFE tube produced by the prior art process was inhomogeneous in diametral dimension and because of the out-of-roundness of each section, when this tube was drawn inside the steel pipe, a diametral clearance between said tube and said steel pipe varied at each point at which the tube and steel pipe were in contact with each other. On that account, a load was applied particularly to the lining tube at the corner of the flange of the steel pipe which was the stop end thereof in the lengthwise direction, and said lining tube was liable to be damaged and, at the same time, the lining tube in use was liable to buckle. Furthermore, because the section of the lining tube was out of round and was somewhat deformed, said lining tube, when drawn into the steel pipe, came to carry an undue burden.
As a measure to inhibit occurrence of such inconveniences as mentioned above, there is known a so-called annealing treatment, wherein the lining tube drawn into the steel pipe and heated, in an electric oven or the like to eliminate warpage formed on said lining tube.
In order to carry out the above-mentioned annealing treatment, special equipment was needed therefor and much labor and energy were required to operate the treatment satisfactorily, leading to a possible increase in production cost. Furthermore, in molded articles obtained by this annealing treatment, it is possible for residual strain to occur in the molded article and for said molded article deterionate in mechanical strength.